James W wrote:

at the 90deg point, the I curve is crossing the zero point. It's a sine
wave. Sine waves of amplitude 1 have a slope of 1 at the zero crossing, no?

No. If the period is 1 and the peak to peak amplitude is 1, the only
way the slope could be 1 would be if it were a saw tooth wave.

--
John Popelish